Background noise suppressor



Feb. 20, 1945.

G. F. DEVINE BACKGROUND NOISE SUPPRESSOR Filed Oct 23, 1943 Inventor":George Fipevine. y Z

His .ttorney Patented Feb. 20, 1945 2,369,952 BACKGRGUND NQISESUPPRESSOR George F. Devine, Easton, Conn assignor to General ElectricCompany, n'eorporation of New York Application October 23, 1943, SerialNo. 507,409

8 Claims. (Cl. 179-400.!)

My invention relates generally to noise sup- Dression circuits for audiofrequency channels of thetype in which the appearance of backgroundnoise cannot be prevented. It is an object of my invention to provide animproved noise suppression circuit for such channels which operatesnormally to remove high audio frequencycurrents only from the output ofthe channel and which is rendered inoperative by the appearance of highaudio frequency currents at the input of the channel to permittranslation of such currents by the channel.

- In audio frequency translating channels which employ electronicamplifiers, it is customary to employ a noise suppression circuit toreduce noise voltages in the output of -the channel in the absence ofsignals to be translated. The time of operation of such circuits is afactor which enters into their successful application. This time must belong compared to the lowest frequency voltages which it is desired totransmit. In order to avoid distortion of the audio signal beingtranslated,

such noise suppression circuits usually operate at syllabic speed. It isan object of my invention to provide an improved noise suppressioncircuit, the suppressor action of which is limited to higher audiofrequencies to permit a very short time constant in the control circuit.

My invention relates more particularly to noise suppression circuits forelectric phonograph record reproducers and it has for one of its objectsto provide an improved circuit for reducing the record background noisein the output of such a reproducer.

The background noise usually encountered in record reproducers is mostannoying during quiet passages of recorded music, when higher frequencytones are not present, and at the beginning and end of records played onautomatic record changers, but is not noticed when the recording levelis high. Accordingly, it is an object of my invention to provide animproved background noise reducing circuit for record reproducers bywhich the high frequency response of the reproducer is reduced whenhigher frequency tones are not present on the part of the record beingreproduced or are below the level of the background noise andis returnedto'normal operation when desirable higher frequency tones, having anintensity greater than that of the backdetermined with a high degree ofaccuracy. Accordingly, a further object of my invention is to provide animproved background noise reducing circuit for a record reproducer inwhich a threshold level for disabling of the noise reducing circuit maybe set or adjusted above the intensity of the high frequency componentsof the background noise which it is desired to suppress and below theintensity of high frequency passages on the recording which it isdesired to reproduce.

One of the features of my invention is the employrnent in a noisereducing circuit for an audio frequency translating channel of alow-pass filter which includes an electron discharge device functioningas a reactance device having a predetermined frequency characteristicand varying the reactance of said device in accordance with theintensity of the higher frequency components of the audio signals at theinput to said channel.

Another object of my invention is to provide an improved noise reducingcircuit for audio amplis fiers which includes an electron dischargereactance device having a determinable frequency characteristic. V

The features of my invention which I believe to be novel are set forthwith particularity in the appended claims. My invention itself, however,both as to its organization and method of operation, together withfurther objects and advantages thereof, may best be understood byreference to the following description taken in connection with theaccompanying drawing, in which Fig. 1 represents a. portion of anelectric phonograph record reproducer embodying the noise suppressioncircuit of my invention, and Fig. 2 is a modification of the circuit ofFig. 1. In the two figures corresponding reference numerals have beenplaced on corresponding elements to of the reproducer amplifying circuitand, ultimately, to a. suitable sound reproducing device. Connectedacross the channel Ill between the terminals ii and I2 is my noisesuppressioncircuit nected in shunt to the crystal Iii to control the lowfrequency response of the pick-up device. Connected between terminal i Iand ground is a high-pass filter circuit comprising capacitance l6 andresistance ii. The common point of capacitance IB and resistance I1 isconnected to control electrode |3 of the triode section of an electrondischarge device I! of the diode-triode type. Anode of the triodesection of device I! is supplied with operating potential through a loadresistance 2| from any suitable source of potential, such as the battery22, and is connected to ground through a capacitor 23 and a resistance24. The cathode of device I3 is connected to ground for unidirectionalcurrents and low audio frequency currents through resistance and forhigh audio frequency currents through by-D S capacitor 26. Thecapacitance of capacitor 28 is small so that only high audio frequencycurrents, i. e. currents having frequencies above approximately 600cycles per second, flow readily through capacitor 26. Capacitance I8similarly is a relatively small capacitance so that the potential ofelectrode It for low frequencies and which is due to the low frequencyaudio currents flowing through resistance H is quite small. Since lowaudio frequency currents flow from the cathode of device iii to groundthrough the high impedance of resistance 25, degeneration occurs and theoutput current of the triode section of device I! contains but a smallcomponent of audio frequencies below about 600 cycles.

Anode 2l of the diode section of device I9 is connected to the commonpoint of capacitor 23 and resistor 24 and to ground through a filtercircuit comprising resistor 28 and capacitor 23.

The audio frequency currents in translating channel In flow through alow-pass filter comprising series resistance 30 and electron dischargedevice 3| functioning as a reactance tube. The anode of device 3| isconnected to battery 22 through a load resistance 33 and to therighthand terminal of resistance 30 through blocking capacitor 34 andcapacitor 35 inserted in the filter circuit to enhance the reactiveeffect of device 3|. The device 3| is shown as a pentode having itscathode connected directly to ground and its control electrode 36connected to the common point of capacitors 34 and 35 and to groundthrough resistance 31 and capacitor 29. The screen electrode 33 issupplied with operating potential from battery 22 through a voltagedropping resistor 33 and is connected to the cathode through acapacitance G0. The electrode II is connected directly to the cathode toact as the usual suppressor grid.

In the operation of the low-pass filter comprising series resistor 30and the electron discharge device 3|, the device 3| functions as awell-known reactance tube having a capacitive reactance which varieswith the gain of the tube. In order that the capacitive reactance of thedevice vary also with the frequency of currents flowing through thatdevice, capacitor 40, connected between screen electrode 38 and thecathode of the device, is made small so that the screen electrodeoperates practically at ground potential for higher audio frequenciesand at potentials considerably above ground for lower audio frequencies.In this manner the device 31 operates as a reactance tube whosereactance varies with the frequency of the currents flowing through thetube. The gain of the device 3| is controlled by the biasing potentialssupplied thereto through resistance 31 from the common point of resstance .23 and capacitance 29 in the filter circuit connected betweenthe anode 21 and ground. The value of resistance 31 is made low, in theorder of 50,000 ohms, so that this resistance not only functions toprevent reactance device 3| from acting to change noticeably theapparent volume level of the output of channel In, as device 3| isrendered operative or inoperative to by-pass high frequency currentsfrom channel In, but also forms in conjunction with capacitor 33 a tonecompensating circuit for the crystal pick-up N. As stated previously,loading resistor l5 reduces the low frequency response of the crystalpick-up and is commonly found necessary in an electrical phonograph toprevent rumble. The compensating action of tone compensating circuit 34,31 is limited to the middle frequencies in the output of pick-up H andreduces these middle audio frequencies in a degree commensurate with theamount that the lowest frequencies are reduced by resistor l5 so as toproduce a pleasing overall tonal quality.

In the operation of the complete noise suppressor circuit i3 high audiofrequency signals are amplified in the triode section of the device I9to provide a potential to the anode 21 which is rectified in the diodesection of the device I9. Low audio frequencies are prevented from beinreproduced in the circuit of anode 20 through the operation of thecathode bias resistor 25 and the high-pass filter circuit i6, IT. Thehigh audio frequency currents are rectified in the diode section ofdevice I! and filtered by resistance 28 and capacitor 29 to provide anegative bias voltage for control electrode 36 which reduces the gain ofdevice 3| and changes its reactive effect to that of a very smallcapacity. When the reactance of the device 3| is thus reduced, highaudio frequencies in the translating channel l0 are permitted to proceedto the terminal l2 without attenuation.

In order to provide a threshold level so that high audio frequencycurrents, present in the channel I0 and due to surface noise alone inthe output of the phonograph record reproducer, cannot reduce the gainof device 3| to permit the high frequency components of this surfacenoise to pass to the output terminal l2, resistance 25 is madeconsiderably larger than required merely for providing a bias voltagefor the trlode section of the device it. In this manner high audiofrequency voltages of a predetermined intensity are required to reducethe by-passing effect of device 3 I. It is apparent, of course, that theflow of current in the diode section of device I9 is delayed also by thevoltage developed across resistance 25 so that the gain reducing biasfor control electrode 38 is developed only after high frequency audiosignals of predetermined intensity are supplied to control electrode l8.

When a crystal pick-up of the type shown is employed in an electricphonograph record reproducer, the noise level of the stylus attached tothat pick-up device and of the average record can be determined withconsiderable accuracy. Also, it is well known that the minimum recordinglevel which can be successfully reproduced can be definitelyascertained. Hence, by properly selecting the value of the cathode biasresistance 25, the operating point of the diode section of device l9 maybe set or adjusted with sufficient precision that high frequency audiotones are by-passed from the translating channel Ill by the reactancedevice 3| when the intensity of audio frequency voltages at terminal IIis below a definite value. In this manner scratchy high frequencybackground noise present on the recording is not carried to the outputcircuits of the phonograph record reproducer during quiet passages inthe recording,.when higher frequency tones are not present, and at thebeginning and end of the records being played. When, however, the highfrequency notes of the record produce voltages at terminal H above athreshold value, biasing potentials are developed by device I! to reducethe gain of reactance tube 3! and permit the passing of these highfrequency tones to the output circuit of the reproducer.

Since only high frequency currents are rectified by the diode section ofdevice ID, the charging time of capacitor 28 can be relatively short sothat negative biasing potential is supplied rapidly to control electrode38 when desirable high frequency voltages appear at the terminal H.Thus, the bypassing effect of reactance device 3! is reduced quiterapidly and the switching off and on of high frequency notes in theoutput circuit of the translating channel occurs at a rate which issubstantially imperceptible to the human ear. This control action. whileseeming almost instantaneous to the ear, remains relatively slow inrelation to the audio frequencies being controlled so that there is nodistortion of these audio frequency notes by the control circuit.

' In the modification of my invention shown in Fig. 2, the screenelectrode 38 of reactance device 3| is by-passed to ground for higheraudio frequencies by the capacitor 40 and the cathode of the device isconnected to ground through a resistance 45 across which is connected aby-Dass capacitor 46. The resistance 45 provides normal bias for device3| as well as degeneration at the frequencies not by-passed by capacitor46. Both capacitors 40 and it have a capacitance value such that theyby-pass only higher audio frequencies. As a result, in this modificationof the noise suppressor circuit, the reactance of the reactance tube 3|is increased for higher audio frequencies to be by-passed from channel land has a very small value for low audio frequencies.

The operation of the circuit of Fig. 2 is essentially the same as thatof the circuit of Fig. 1. The main difference between the two circuitslies in the fact that low frequency currents flowing through device 3|are reduced to a very small value by the degenerative voltages developedacross cathode biasing resistor l5 and the high impedance of capacitors40 and 46 to these low audio frequency currents. on the other hand. forhigher audio frequency currents the cathode and screen electrodes of thedevice operate practically at ground potential and the gain of thedevice is controlled by the potential of control electrode 35.

By way of illustration only, and not in any sense by way of limitation,the following representativc values are given for a circuit constructedin accordance with Fig. 1 of my invention. A 6SQ7 type of electrondischarge device was used as the device I! with the two diode-anodes ofthe device being connected together to comprise the anode 21 and a type6SG7 tube was used as the reactance tube 3|. The values of the impedanceelements of the circuit are as follows:

Capacitor I 6:.001 microfarad Capacitor 26:.1 microfarad Capacitor29:.05 microfarad Capacitors 35, 40:.01 microfarad Capacitor 34:.00025microfarad While I have shown a specific embodiment of myinvention, itis apparent that various changes may be made. Thus, instead of using thebias resistance 25 to obtain a threshold level for the two sections ofthe device IS. a bias voltage may be used. Similarly, when the audiofrequency voltages at the terminal II are or high level, or areamplified before passing through the series resistance 30, it isdesirable to use. a greater bias for both tubes in the device 3|.However, in all applications of the noise suppressor circuit, it isdesirable that the circuit be applied to stages of an amplifierpreceding the volume control means in the amplifier. In this way thecircuit remains independent of the output level at which the reproduceris operated and rapid, "reliable, and accurate operation of thesuppressor circuit is obtained.

While I have shown a particular embodiment of my invention, it will ofcourse be understood 7 that I do not wish to be limited thereto sincevarious modifications may be made, and I contemplate by the appendedclaims to cover any such modifications as fall within the true spiritand scope of my invention.

What I claim as new and desire to. secure by Letters Patent of theUnited States, is:

1. In combination, a channelfor translatin audio frequency signals,means coupled to said channel for attenuating the high audio frequencysignals therein while permitting translation of lower audio frequencysignals, and means responsive to the presence of intense high audiofrequency signals in said channel for reducing the attenuation of saidsignals by said attenuating means. a q

2. In combination, a translating channel for audio frequency signals,means coupled to said channel for substantially removing. audio signalshaving a frequency greater than a predetermined frequency from theoutput of said channel while permitting translation of lower audiofrequency signals, means responsive to the appearance of audio signalsof a frequency greater than said predetermined frequency in the input tosaid channel to disable said signal removing means, and means to providea threshold level for the operation of said disabling means.

3. In combination, a background noise reducing circuit for the audiofrequency translating circuit of a record reproducercomrprising, acapacitance and a resistance connected in series across said audiocircuit, and an electron discharge device connected in shunt to saidresistance, said device having a control electrode and said noisereducing circuit being adjusted to. remove undesired high frequencycurrents from said audio circuit, and means to disable said noisereducing circuit to permit translation of desired high frequencycurrents comprising, means to rectify high frequency voltages present insaid audio circuit, and means for applying said rectified voltages tosaid electrode in a gain reducing sense.

4, In combination, a surface noise reducing circuit for the audiofrequency translating circult of a record reproducer comprising, 8.capacitance and a. resistance connected in series across said audiocircuit, and an electron discharge device connected in shunt to saidresist once, said device having a control electrode and said noisereducing circuit being adjusted to remove high frequency currents havingan intensity less than a predetermined intensity from said audiocircuit, and means to disable said noise reducing circuit to permittranslation of high frequency currents of an intensity greater than saidpredetermined intensity comprising, means to rectify high trequencyvoltages present in said audio circuit, and means for applying saidrectified voltages to said electrode in a gain reducing sense.

5. In a filter circuit for an audio frequency translating channel, thecombination of, an input terminal, and an output terminal, an inputcircuit connected between said input terminal and a point of fixedpotential, an output circuit connected between said output terminal andsaid point, a resistance connected between said terminals, an electrondischarge device having an anode connected to a point between saidresistance and said output terminal, a cathode connected to said pointof fixed potential, a screen electrode and a control electrode, saidscreen electrode being connected to said point or fixed potentialthrough a capacitance having a large reactance at low audio frequenciesand a small reactance at high audio frequencies, and means to bias saidcontrol electrode in accord with the intensity of high audio frequencyvoltages across said input circuit.

6. In a filter circuit for an audio frequency translating channel, thecombination of, an input terminal, and an output terminal, an inputcircuit connected between said input terminal and a point of fixedpotential, an output circuit connected between said output terminal andsaid oint, a resistance connected between said terminals, an electrondischarge device having an anode connected to a point between saidresistance and said output terminal, a cathode, a screen electrode, anda control electrode, said screen electrode being connected to said pointof fixed potential through a capacitance having a large reactance at lowaudio frequencies and a small reactance at high audio frequencies, meansto bias said control electrode in accord with the 40 intensity of highaudio frequency voltages across said input circuit, and degenerativemeans for low audio frequency currents connected between said cathodeand said point of fixed potential.

'1. In a filter circuit for an audio Irequency translating channel, thecombination or, an input terminal, and an output terminal, an inputcircuit connected between said input terminal and a point 0! fixedpotential, an output circuit connected between said output terminal andsaid point, a resistance connected between said terminals, and avariable reactance connected across said output circuit comprising anelectron discharge device having an anode connected to a point betweensaid resistance and said output terminal and a cathode connected to saidpoint of fixed potential, and means to vary the reactance of said devicein accordance with the frequency of the current flowing through saiddevice.

8. In a filter circuit for an audio frequency translating channel, thecombination of, an input terminal, and an output terminal, an inrputcircuit connected between said input terminal and a point oi fixedpotential, an output circuit connected between said output terminal andsaid point, a resistance connected between said terminals, a variablereactance connected across said output circuit'comprising an electrondischarge device having an anode connected to a point between saidresistance and said output terminal and a cathode connected to saidpoint of fixed potential, means to vary the reactance of said device inaccordance with the frequency of the current flowing through saiddevice, and means to vary the gain 01 said device in accordance with thefrequency of the currents in said input circuit.

GEORGE F. DEVINE.

DISCLAIMER 2,369,952.-George F. Davina, Easton, Conn. Bacxonormn Nolansurranssoa.

Patent dated Feb. 20, 1945. assignee, General Electric Company.

Disclaimer filed Sept. 20, 1945, by the Hereby enters this disclaimer toclaims 1 and 2 of said patent.

[Oflicial Gazette October 30, 1945.]

than said predetermined intensity comprising, means to rectify hightrequency voltages present in said audio circuit, and means for applyingsaid rectified voltages to said electrode in a gain reducing sense.

5. In a filter circuit for an audio frequency translating channel, thecombination of, an input terminal, and an output terminal, an inputcircuit connected between said input terminal and a point of fixedpotential, an output circuit connected between said output terminal andsaid point, a resistance connected between said terminals, an electrondischarge device having an anode connected to a point between saidresistance and said output terminal, a cathode connected to said pointof fixed potential, a screen electrode and a control electrode, saidscreen electrode being connected to said point or fixed potentialthrough a capacitance having a large reactance at low audio frequenciesand a small reactance at high audio frequencies, and means to bias saidcontrol electrode in accord with the intensity of high audio frequencyvoltages across said input circuit.

6. In a filter circuit for an audio frequency translating channel, thecombination of, an input terminal, and an output terminal, an inputcircuit connected between said input terminal and a point of fixedpotential, an output circuit connected between said output terminal andsaid oint, a resistance connected between said terminals, an electrondischarge device having an anode connected to a point between saidresistance and said output terminal, a cathode, a screen electrode, anda control electrode, said screen electrode being connected to said pointof fixed potential through a capacitance having a large reactance at lowaudio frequencies and a small reactance at high audio frequencies, meansto bias said control electrode in accord with the 40 intensity of highaudio frequency voltages across said input circuit, and degenerativemeans for low audio frequency currents connected between said cathodeand said point of fixed potential.

'1. In a filter circuit for an audio Irequency translating channel, thecombination or, an input terminal, and an output terminal, an inputcircuit connected between said input terminal and a point 0! fixedpotential, an output circuit connected between said output terminal andsaid point, a resistance connected between said terminals, and avariable reactance connected across said output circuit comprising anelectron discharge device having an anode connected to a point betweensaid resistance and said output terminal and a cathode connected to saidpoint of fixed potential, and means to vary the reactance of said devicein accordance with the frequency of the current flowing through saiddevice.

8. In a filter circuit for an audio frequency translating channel, thecombination of, an input terminal, and an output terminal, an inrputcircuit connected between said input terminal and a point oi fixedpotential, an output circuit connected between said output terminal andsaid point, a resistance connected between said terminals, a variablereactance connected across said output circuit'comprising an electrondischarge device having an anode connected to a point between saidresistance and said output terminal and a cathode connected to saidpoint of fixed potential, means to vary the reactance of said device inaccordance with the frequency of the current flowing through saiddevice, and means to vary the gain 01 said device in accordance with thefrequency of the currents in said input circuit.

GEORGE F. DEVINE.

DISCLAIMER 2,369,952.-George F. Davina, Easton, Conn. Bacxonormn Nolansurranssoa.

Patent dated Feb. 20, 1945. assignee, General Electric Company.

Disclaimer filed Sept. 20, 1945, by the Hereby enters this disclaimer toclaims 1 and 2 of said patent.

[Oflicial Gazette October 30, 1945.]

